Protein phosphorylation is the ubiquitous strategy used to control the activities of eukaryotic cells. It is estimated that 10% of the proteins active in a typical mammalian cell are phosphorylated. The high energy phosphate which confers activation and is transferred from adenosine triphosphate molecules to a protein by protein kinases is subsequently removed from the protein by protein phosphatases. In this way, the phosphatases control most cellular signaling events that regulate cell growth and differentiation, cell-to-cell contacts, the cell cycle, and oncogenesis.
The inositol polyphosphate phosphatases include several monomer enzymes of different molecular weights (115-160 kD, 69-75 kD, and 32-43 kD) which hydrolyze inositol polyphosphates. Such phosphatases are known to remove the phosphate from the 3, 4, or 5 position of the inositol ring of several different substrates. For example, membrane-bound polyphosphoinositide phosphatase from rat brain utilizes phosphatidylinositol (4)phosphate, phosphatidylinositol (3)phosphate and phosphatidylinositol 4,5-bisphosphate (PIP2) as substrates. Phosphatidyl-inositol 4,5-bisphosphate 5 phosphatase preferentially removes the position 5 phosphate from PIP2, but its affinity for other positions or substrates is not fully known (Hope H. M. and L. J. Pike (1994) J. Biol. Chem. 269:23648-54; Jefferson and Majerus (1995; J. Biol. Chem. 270:9370-9377).
Hydrolysis of PIP2 produces diacylglycerol (DAG) and inositol triphosphate (IP3), both of which act as second messengers in the cell signaling pathways. DAG and IP3 release Ca2+ from intracellular stores (endoplasmic/sarcoplasmic reticulum) and promote Ca2+ influx from the extracellular fluid. The phosphatases that catalyze the removal of phosphate are positively and negatively regulated by the local concentration of various ions including CA.sup.-, Mg.sup.-, and Li.sup.-.
Jefferson and Majerus (supra) expressed and studied a type II polyphosphate 5 phosphatase cloned from a human erythroleukemia cell cDNA library. Their protein was 942 amino acids in length; hydrolyzed inositol 1,4,5-triphosphate, inositol 1,4-bisphosphate, and PIP2; shared two potential binding/catalytic motifs (amino acids 472-483 and 545-570) with other phosphatases; and had a 3'CPNL motif that suggested isoprenylation and membrane association. They showed that antibodies raised against a recombinant 75 kD inositol (1,4,5) triphosphate 5 phosphatase also depleted phosphatidyl-inositol 4,5-bisphosphate 5 phosphatase activity from the cytosolic and membrane fractions of platelets; and they suggested that tissue specific expression and/or proteolytic processing of the larger phosphatases may be the source of the cytosolic phosphatases.
The discovery of a new human phosphatidylinositol 4,5-bisphosphate 5-phosphatase and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of disorders of cell proliferation and cell signaling.